You learn where your customers hang out, and then you hang out there and authentically become part of the social group. You spend weeks/months/years learning what they like, their workflows, and their problems. Or, better, you've already spent years working with your customers so you have some experience in these matters. Then you spend more time setting up face-to-face discussions, at conferences, online, or wherever they hang out, *not* trying to shill, but honestly and authentically trying to learn what they need.
It may take dozens or hundreds of attempts, and then you find a small group, maybe a half dozen or so, that are early adopters, willing to live with your experiments and provide feedback. Work with them to hone the value proposition, and learn how to communicate it effectively. Tweak or pivot the product to fit their needs, often for many more months.
There is no simple solution that involves making a few social media posts, or paying for advertisements, or spamming people with email. Everything that actually works takes lots of personal time and energy.
Has this ever worked for you? If so, could you walk us through your journey?
In my experience if you spend a really long time trying to identify a problem to solve, you end up burning too much time on a problem that may not work. The Indiehackers approach is like the opposite of that where you shotgun low-effort attempts at ideas until you find one that sticks. I think most folks trying to build a business want something in the middle though. Use your experience and your knowledge to winnow the market of potential opportunities and to offer you an advantage (with your expertise) then iterate by creating different products until one of those products gets traction.
When I worked at a tech company that eventually became one of the Big Tech Unicorns of the last rush, we had plenty of products that completely bombed, much to the sadness of the folks that worked on them.
This approach has worked for me, and many others. Read the Lean Startup for the general approach, and The Mom Test to learn how to talk to prospective customers. The main point is that you need to talk to customers and work with them throughout the process.